Ch 8 Quiz: Icing — Types, Aircraft Icing & Carburettor Icing

Main airframe icing types: RIME ice (opaque, white, brittle — forms in small supercooled droplets, fast freezing) and CLEAR (glaze) ice (transparent, dense, heavy — large droplets, slow freezing). Mixed icing also occurs.

Frost and hoarfrost

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Rime ice and clear (glaze) ice

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Structural icing and engine icing

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Contact icing and evaporative icing

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Clear ice: forms from large supercooled droplets or freezing rain that spread before freezing. Heavy, transparent, adheres strongly to surfaces, extremely difficult to remove. Causes greater aerodynamic disruption than rime. Most dangerous icing type.

It is heavier

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It is heavy, hard, adherent, transparent (hard to see), and conforms closely to the airfoil shape — most aerodynamic penalty

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It forms only at low altitudes

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It forms faster

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Structural icing is most intense between 0°C and −20°C where supercooled liquid water droplets exist in greatest concentrations. Below −20°C most water is frozen (ice crystals which do not readily adhere). Below −40°C all water is frozen.

0°C to −5°C

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0°C to −20°C (peak risk −2°C to −10°C, liquid water content highest)

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−20°C to −40°C

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Below −40°C

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SLD (freezing drizzle and freezing rain droplets > 50 μm): run back beyond leading-edge deicing protection before freezing, coating upper wing surfaces and tail. SLD conditions can exceed certification limits of anti/deicing systems — extra hazard.

They cause engine damage only

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They impinge on aircraft surfaces beyond the protected zone (boots/bleed-air leading edges), degrade aircraft performance severely

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They freeze immediately

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They only form over oceans

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Carb icing: caused by evaporative cooling of fuel + adiabatic cooling in the venturi, which can drop air temperature 20–30°C. Can form at ambient temperatures up to +30°C in humid conditions. Most insidious at low power (descent).

Temperature is below −20°C

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Temperature is between −10°C and +25°C with high relative humidity — even on a warm day

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Only in cloud

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Temperature is above 30°C

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Freezing rain: rain formed above (warm air) falls into sub-zero air below the frontal surface. Large supercooled droplets instantly glaze exposed surfaces with clear ice. Associated with the classic warm frontal warm-rain process over cold sub-frontal air.

It forms only near thunderstorms

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It is supercooled rain (large drops) that instantly forms clear ice on contact — associated with warm fronts and inversions

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It causes windscreen icing only

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It freezes propellers but not wings

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Below approximately −20°C most water droplets have frozen to ice crystals which are less adhesive (though ice crystal icing at engines is still a risk). Risk is very low below −40°C. UK CAA/EASA uses −40°C as the lower icing bound for certification.

0°C

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−10°C

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−20°C to −40°C (FAA rule of thumb −20°C, EASA −40°C)

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−5°C

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Tailplane (horizontal stabiliser) ice stall: ice builds on the tailplane, disrupting its downwash and lift. Extending flaps increases downwash angle, triggering abrupt pitch-down. Recovery: RETRACT flaps, increase speed. Opposite recovery to wing stall.

Stick shaker activation

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Uncommanded pitch-down, elevator buffet, and loss of pitch control — often triggered by flap extension

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Engine vibration

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Rudder hardover

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Pitot/static icing blocks the pitot tube (false ASI), static ports (false ALT/VSI/ASI), and can cause dangerous erroneous instrument indications. Multiple accidents have resulted from blocked pitot/static systems (e.g. Air France 447).

Wing lift only

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Airspeed indicator, altimeter, and VSI — can give false readings leading to controlled flight into terrain (CFIT) or upset

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Engine performance

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Compass only

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Engine inlet icing: ice in the intake reduces airflow, causes distorted airflow to compressor, vibration, and potential compressor stall or flameout. Jet engines use bleed air or electrical anti-icing on inlet lips. Piston engines use carb heat or alternate air.

Fuel flow only

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Airflow to the engine — can cause power loss, compressor stall, or engine flameout

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The exhaust only

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Hydraulics

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Anti-icing: prevents ice formation (heated leading edges, windshield heat — must be ON before entering icing). De-icing: removes ice after accumulation (pneumatic boots — cycle to break off ice ridges). Both are critical; confusion can lead to improper use.

Also prevent ice formation

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Remove ice after it has formed

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Are the same thing

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Only work on propellers

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Worst icing: Cumulonimbus and large cumulus congestus (very high liquid water content, large droplets), and freezing rain/drizzle layers under warm fronts. Nimbostratus and altostratus also produce significant icing in the 0°C to −20°C range.

Cirrus clouds

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Cumulus/convective clouds (high LWC) and freezing rain layers

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Dry snow storms

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Stratocumulus above FL200

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Icing intensity: TRACE ( 1.25 mm/hr, de-icing inadequate — exit immediately). PIREPs use this scale.

Density only

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Rate (thin, light, moderate, severe — based on accumulation rate per hour)

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Weight per square metre

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Temperature differential

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Icing SIGMETs are issued for moderate or severe icing expected within a defined area/altitude block over a specified time period. They are based on pilot reports (PIREPs), radar, satellite, and NWP model output.

Any cloud is present

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Moderate or severe icing is expected in a defined area, based on forecast or PIREP

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Temperature is below 0°C anywhere

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Frost is expected at airports

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Ground icing (‘cold soak’ frost, freezing rain on parked aircraft): even a thin layer of frost (0.5 mm) on a wing can reduce lift by 30% and increase stall speed significantly. Aircraft must be de-iced/anti-iced (‘clean aircraft concept’) before take-off.

It causes fuel contamination

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Even thin frost disrupts laminar flow over wing, degrading lift and stall characteristics — contributes to many take-off accidents

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It blocks the pitot only

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It increases weight to unacceptable levels

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Type IV fluid (yellow/green): highly viscous, provides longest holdover time (up to ~3 hours in light freezing drizzle). It shears off cleanly during take-off rotation. Holdover time tables are published by SAE and must be consulted before flight.

It is water-based

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It is a thick, tenacious anti-icing fluid that resists being blown off during take-off roll

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It is heated to high temperatures

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It is the cheapest fluid

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Ice contamination: increases drag (up to 40%), reduces lift (up to 30%), raises stall speed, causes unpredictable stall characteristics (buffet may be reduced or absent), and increases weight. Even frost-roughened surfaces significantly degrade performance.

Increase lift

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Decrease lift, increase drag, raise stall speed, and alter stall characteristics — even 0.5 mm of roughness is significant

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Increase thrust

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No effect below −10°C

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Runback ice: water from melted icing accumulates at the edge of the heated deiced zone, flows aft (due to aerodynamic forces), and refreezes on the unprotected wing surface behind the leading edge. Creates a ridge of clear ice aft of the boot/heater.

Ice melting off the wings

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Water that flows aft of the heated/deiced leading edge and refreezes on an unprotected surface

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Ice pellets bouncing off the airframe

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Hail impact damage

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Large turbofan engine inlets: the wide inlet area of high-bypass engines collects substantial ice. Ice can shed and enter the core, causing Fan Blade Damage, compressor stall, or flameout. Engine anti-ice (bleed air or electric) is critical in icing conditions.

Hydraulics

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High-bypass turbofan engines — large inlet area and sensitive compressor stages are susceptible to ice accretion and shedding causing compressor stall

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Piston engine exhaust

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Fuel system

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ICI: ice crystals (not supercooled liquid water) ingested into engines, particularly in jet engines. Crystals melt on warm internal surfaces, causing ice accretion on fuel nozzles and guide vanes. Detection is difficult — no airframe icing, no weather radar return. Affects engines at high altitude.

In low-level stratus

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In regions of high ice water content (HIWC) in deep convection (Cbs, MCSs) at high altitude — particularly near the tropics

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In freezing rain at low altitude

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Over polar ice caps

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Anti-icing is PREVENTIVE — it must be active BEFORE ice starts to form. Once icing has formed on a surface without anti-icing, the built-up ice must be removed by de-icing. The key distinction: anti-ice (prevent) vs. de-ice (remove).

After ice forms

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BEFORE entering icing conditions — the anti-icing system prevents formation rather than removes existing ice

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Only in flight

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On the ground only

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Holdover time (HOT): the estimated time during which de-icing/anti-icing fluid will protect the aircraft from ice accumulation under specific weather conditions. Published in tables based on: fluid type, outside air temperature (OAT), and precipitation type/rate.

Time to apply the fluid

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The estimated time after fluid application during which the fluid retains its ability to prevent ice formation on surfaces

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The shelf life of the fluid

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The time needed for the fluid to work

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Type I: heated, orange, diluted glycol mixture. Low viscosity (flows off immediately on take-off). Used mainly for de-icing (removing existing ice/snow/frost). Provides very short HOT — typically 3–8 minutes in active precipitation.

The most protective type

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A heated, low-viscosity fluid (orange) used primarily for de-icing — removes existing ice but provides minimal holdover time (

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Used for large aircraft only

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A runway de-icer

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Clean aircraft concept: ALL contamination (frost, ice, snow) must be removed from critical surfaces (wings, tailplane, fuselage behind engines) before take-off. There is NO safe amount of frost on a wing. This principle is supported by numerous accident investigations.

Accepted if thin (

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Completely removed — even thin frost disrupts laminar flow, reducing lift and increasing stall speed ('clean aircraft concept')

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Melted in flight

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Wiped with a dry cloth

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FZRA = Freezing Rain. FZ (freezing) + RA (rain). Large supercooled rain drops that immediately form clear ice on contact with sub-freezing surfaces. A severe icing hazard — can accumulate ice very rapidly (>1 mm/min). Associated with warm fronts and temperature inversions.

Freezing rain — supercooled large droplets that glaze surfaces with clear ice

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Falling zero rain

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Fog and rain

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Freezing drizzle only

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Pneumatic boots: rubber tubes along the leading edge, inflated with compressed air (alternating cycles). Inflation breaks the ice shell that has formed. They are DE-icing systems (remove formed ice). They must NOT be inflated too early (before ice builds up) or ice may ‘bridge’ over them.

Heating the leading edge

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Expanding inflatable rubber tubes that break off the ice ridge that has formed — must cycle periodically

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Dissolving ice chemically

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Vibrating the wing surface

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Ice stall: ice on the wing changes the pressure distribution, reduces CL_max, and can cause earlier boundary layer separation. The stall occurs at a lower angle of attack (unexpectedly) and a higher airspeed. Stall warning (stick shaker) may activate too late or not at all.

Ice adds fuel

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Ice disrupts the smooth flow over the wing, reducing the effective camber and maximum lift coefficient — a higher angle of attack and thus higher speed is needed to maintain lift

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Ice reduces drag

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Ice increases density altitude

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Tailplane icing: the horizontal stabiliser has a shorter chord and thinner profile than the wing, making it more susceptible to icing. Ice disrupts downwash, reducing the stabiliser’s ability to provide nose-up pitching moment. Flap extension increases the local angle of attack → stall.

Wing leading edge

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Horizontal stabiliser (tailplane) — less protected, smaller chord, and critical for pitch stability; ice can cause tailplane stall on flap extension

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Rudder

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Engine cowls

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Appendix C (CS-25/FAR 25 Appendix C): the standard icing certification envelope for transport-category aircraft. Covers: temperature (0°C to −40°C), liquid water content (LWC), and median volumetric droplet diameter (MVD). Appendix O covers SLD conditions.

All icing conditions

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Supercooled liquid water conditions: continuous maximum (stratiform) and intermittent maximum (convective) icing — specific LWC, MVD, and temperature ranges

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SLD and ice crystals only

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No certification standard exists

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Severe icing (AMS/PIREP definition): ice accumulates faster than de-icing equipment can handle. Immediate exit from icing conditions is required. This may mean descent to warmer air, climb above icing layer, or heading change. Do NOT continue into severe icing.

Slow accumulation over hours

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Ice accumulates so rapidly that de-icing/anti-icing systems are ineffective — immediate action (altitude/heading change) is required

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Moderate rate manageable with boots

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Temperature below −40°C

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PIREP icing: /IC (intensity)(type). E.g. /IC MOD CLR = moderate clear ice. Intensity codes: TRACE, LGHT, MOD, SEV. Type: RIME, CLR (clear), MXD (mixed). Always include FL, OAT, and aircraft type for usefulness to other pilots.

Aircraft registration only

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TB (turbulence) not icing — icing is reported as IC in PIREPs followed by intensity (TRACE/LGHT/MOD/SEV) and type (RIME/CLR/MXD)

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ATC frequency

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Flight time only

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Icing adhesion: supercooled LIQUID water (in Sc, As, Ns) strikes and immediately freezes to metal surfaces. ICE CRYSTALS (in Ci) have much lower adhesion — they bounce off rather than sticking. This is why the main icing zone is 0°C to −20°C where liquid water is most abundant.

Sc is colder

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Sc is composed of supercooled liquid water droplets (at 0°C to −20°C) that adhere to surfaces; Ci is ice crystals at −40°C which do not readily adhere

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Sc is higher

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Sc has stronger turbulence

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Icing awareness: check OAT (0 to −20°C is primary zone), visible moisture (cloud, precipitation, freezing fog), and aircraft certification. Note: icing can occur IN FLIGHT even in cloudless air if flying just below cloud base where supercooled drizzle falls, or in freezing precipitation.

Ice forms only in cloud

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Always check for icing conditions in preflight — if OAT is 0 to −20°C and moisture is present (cloud, precipitation, fog, high humidity), icing is possible

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Icing only occurs above FL100

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Only jet aircraft are affected

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Windshield anti-icing: (1) electrically heated windshield panels (most jets) or (2) hot bleed air directed over the outer surface. Both prevent ice formation rather than removing it after formation. Wiper + rain repellent complement but do not substitute for anti-icing.

Cabin heat only

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Electrical or hot air anti-icing of the windshield panels — prevents ice formation and maintains forward visibility

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Windscreen wipers only

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Defrost only on the ground

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